1. Field of the Invention
The present invention pertains to a detector system and method for determining the rate of sand or other particulate solids production or flow in a fluid flowstream, particularly for determining the sand or proppant production rate in an oil or gas well.
2. Background
Various techniques have been developed for determining the presence of particulate solids such as sand in a fluid flowstream. One area of endeavor in which the detection of sand volume or mass flow rate or fracture proppant flow rate is important is in oil and gas production. The monitoring of sand or fracture proppant flow is critical to minimizing excess wear on fluid piping and other flow control components and to control proper production rates from a subterranean reservoir.
U.S. Pat. Nos. 3,841,144 to Baldwin; 3,854,323 to Hearn et al; 4,240,287 to Mast et al; and 4,674,337 to Jonas describe examples known to us representing the prior art in acoustical sand detector devices and methods. An article entitled "Acoustic Measurements Detect Sand in North Sea Flowlines" by Folkestad et al, Oil and Gas Journal, Aug. 27, 1990, and a paper entitled "New Instrumentation for Managing Sand Problem Prone Fields" by Stuivenwold et al, SPE No. 9368 by the American Institute of Mining, Metallurgical and Petroleum Engineers, Inc., Sep. 21-24, 1980, also describe acoustic sand detection methods. The Baldwin patent describes an acoustic type detection probe which may be inserted in a fluid flowline and which transmits particle impacts to a piezoelectric type sensor, the output of which is filtered and rectified and compared with a reference signal to provide an output signal indicating the presence of sand in the flowline. The Hearn reference measures sand concentration in a flowline with an acoustic detector wherein the output signal is amplified at two frequencies and resultant signals are combined electronically in such a way that the output signal is a function of sand concentration and nearly independent of fluid flow velocity. The Mast et al patent also describes an acoustic type sand detector wherein the output signal from the detector probe is subjected to a signal pulse height discriminator and pulse counter to determine sand or other particulate solids flow rates. The Jonas patent describes an arrangement of a sand detector probe for a fluid flowline together with noise compensating probes, also attached to the flowline, for subtracting extraneous signal noise from the output signal of the sand detector probe.
The accuracy of the prior art sand detectors described in the above-noted references has not been sufficient to make these devices acceptable commercially in many applications. Accordingly, there has been a continuing need to develop a more accurate sand detector which can distinguish the occurrence of sand or other particulate solids flow in a fluid flowstream and can also accurately determine the flow rate or concentration of sand or particulate solids in the fluid flowstream. The present invention provides a system and method which meets these desiderata.